I don't think the firewall is going to be of any help, and the best I can come up with, for the moment, works with a macro in Outlook that intercepts Send and looks for a 'yes/no' custom document property in the document attachment named "Reserved Access" with a value of 'True'. If it finds that in any attachment when the user sends the file then the message is not sent and the user sees a warning. The macro goes in the ThisOutlookSession module, but while it works if all conditions are right, there are no end of loopholes. PDF format is another issue entirely.
Option Explicit
Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
Dim olAtt As Attachment
Dim wdApp As Object
Dim oDoc As Object
Dim oProp As Object
If Item.Attachments.Count > 0 Then
For Each olAtt In Item.Attachments
If olAtt.FileName Like "*.do*" Then
olAtt.SaveAsFile Environ("Temp") & Chr(92) & olAtt.FileName
On Error Resume Next
Set wdApp = GetObject(, "Word.Application")
If Err Then
Set wdApp = CreateObject("Word.Application")
End If
On Error GoTo 0
Set oDoc = wdApp.documents.Open(Environ("Temp") & Chr(92) & olAtt.FileName)
For Each oProp In oDoc.CustomDocumentProperties
If oProp.Name = "Reserved Access" Then
If oProp.Value = True Then
MsgBox "You may not send the attachment " & olAtt.FileName
Cancel = True
End If
Exit For
End If
Next oProp
oDoc.Close 0
Kill Environ("Temp") & Chr(92) & olAtt.FileName
End If
Next olAtt
End If
lbl_Exit:
Set wdApp = Nothing
Set olAtt = Nothing
Set oDoc = Nothing
Set oProp = Nothing
Exit Sub
End Sub